This invention relates generally to electrophotographic imaging and more particularly, provides improved apparatus and methodology for electrophotographically producing color proofs for either direct or offset transfer from color separated transparencies.
Color proofs are needed to show the printing craftsman the results of color separation and whether the corrected separations are suitable for plate making. Of considerable importance is the simulation or prediction of the appearance of the final printed copy on the particular medium used for the final print-run. Proofs are especially needed at two stages in the printing process and are divided into two primary groups, i.e., separation proofs and pre-press proofs.
Separation proofs are made directly by the photoreproduction apparatus to determine the results of the separation process and the identity and character of any corrections needed. Of considerable importance is the capability of accurate and reproducible evaluation of factors such as color balance, tone reproduction, shadow detail, image sharpness, and contrast, among others. Economy and speed in making such proofs are sought after goals in color proofing. Equally important are reliability, reproducibility and predictability. The proof must reproduce the color separation film exactly without distortion or loss. Exact replicas of the printing ink characteristics should be reproduced so that overprinting colors will be the same on the proofs as they are with printing inks employed on the printed sheet.
Pre-press proofs are made from the final corrected separation transparencies. The pre-press proof is intended to reproduce the result which will be obtained using the printing press, indicating the effects of the paper surface, ink strength, gloss, etc. The pre-press proof should show the same printing characteristics as the finished printed result.
The paper surface has an important effect on the appearance of the finished print and, in particular, the critical characteristics of said surface which affect the resultant print are color, ink absorbency and gloss. Color proofs can be made which simulate the effects of paper color. The effects of ink absorbency and gloss are complex and difficult to duplicate. Prints on newsprint lack contrast, are muddy in the middle tones and the inks applied thereto are dull. Prints on uncoated papers have improved contrast compared to prints on newsprint but the inks are still dull with middle tones dark and shadows lacking detail. Coated papers also result in different contrast, gloss, tone characteristics, etc. Thus a proof should be made on the actual paper which is to constitute the substrate carrying the finished printed image.
Several photomechanical processes for prepress-proofing are available. These systems fall into two categories, namely, overlay systems and superimposition systems.
Overlay systems consist of a set of transparent light sensitive films which are dyed or pigmented to simulate the four process colors, yellow, cyan, black and magenta. Each screened separation is exposed to the appropriate film and developed chemically. After development, four separate images are produced which are superimposed in register. The result is viewed as a transparency. These are generally employed where a quick and inexpensive proof is required and normally are not a satisfactory match for the printed reproduction. The whites are gray and the result, very glossy, suffering from internal reflections between film layers which generally cause color changes in overprinted colors. They are economical to produce, require no special equipment and are extensively used for internal checking.
Superimposition systems involve the production of an image on an integral backing sheet either specific to the process or of the type on which the final print will be made. These processes include the Cromalin process of DuPont Co., the Transfer Key process of Minnesota Mining and Manufacturing Corporation, the Gevaproof process of Agfa-Gevaert and the Remak process of Chemical Corporation of Australia, Pty. Ltd.
The Cromalin process involves the lamination of a tacky, transparent, photopolymer film to a base sheet under heat and pressure. The film is hardened by exposure to ultraviolet light. The protective cover sheet is removed and toning powder of the appropriate color is dusted over the surface. The toner adheres only to the areas where no exposure has been received and the polymer remains tacky. The proof is produced by repeating this procedure four times, once for each separation. The base material is a heavy kraft, coated paper or a paperboard member, thus requiring specially made stock.
The Transfer Key process can employ any base stock. A set of four transparent light sensitive films are supplied which have been pigmented to simulate the four process colors. These films are coated with a pressure sensitive adhesive and may be adhered to a base stock to form the laminate. The exposed image is polymerized by exposure to ultraviolet light. The unhardened areas are removed by a solvent with the proof being built up one layer at a time. This process can be improved by producing the layers on a transparent base which in turn is laminated to a base sheet using a spacer to simulate dot gain.
The Gevaproof process also uses laminations to a base stock similar to the Transfer Key process.
The REMAK process is an electrostatic process wherein a sheet of paper coated with a zinc oxide/resin binder composition is charged electrostatically and exposed to light through a color separated transparency. The exposed sheet is immersed in a liquid toner bath and electrophoretically toned. The resulting visible image is transferred to any base stock or, alternatively, the proof may be built up by successive exposures and toning on the original base material. Unfortunately, the zinc oxide photoconductor used with the REMAK process is extremely sensitive to changes in temperature and relative humidity, as well as variations in toner lots.
Many of the problems of prior art proofing methods have been solved by the invention of U.S. Pat. No. 4,358,195. This patent discloses a method and apparatus which takes advantage of the high speed response of an electrophotographic member using a flat-bed machine having plural stations sequentially arranged linearly along a framework. A color separation transparency is mounted on a copyboard and presented to a charged electrophotographic member; the transparency is superposed and exposed to a light source. The carrier for the electrophotographic member was manipulated, i.e., pivotally inverted, and presented to a movable toning station. The toned member is again inverted for presentation to a transfer means effective to transfer the toned image to a sheet of print stock. The process can be repeated with different separations and toners with registration being obtained by registration means provided for properly positioning both the color separation and electrophotographic medium.
Another improved apparatus and method is provided by the invention disclosed in pending patent application Ser. No. 348,769 filed Feb. 16, 1982 and assigned to the same assignee as the subject application. The referenced application discloses electrophotographic imaging apparatus, particularly for color proofing, that is provided for normal daylight operation and includes: a light-tight housing having a framework mounting plural functional processing stations comprising, a charging station, an imaging or exposure station, a toning station, an image transfer station and a cleaning station, all interior of the housing. The apparatus of said pending application is primarily for producing color proofs for direct transfer technology.
Methods are known in which a developed electrostatic latent image is transferred to an intermediate member and subsequently transferred therefrom to an image receiving member. U.S. Pat. No. 3,862,848 provides an example of prior electrophotographic systems wherein the developed electrostatic latent image is transferred to the working surface of an intermediate member and subsequently is transferred to an image receiving surface. The intermediate member has an electrically conductive working surface which facilitates both the first and second transfer steps by the concurrent physical contact and application of an electrostatic field.
U.S. Pat. No. 4,182,266 describes an offset mode of transfer wherein an image deposit is transferred to an intermediate or offset member and thereafter, transferred to a transfer receiving member. The transfer steps are effected by the simultaneous contact and application of an electrostatic field. This patent describes a method and means for effecting transfer to a flexible or rigid transfer receiving member, such as metal plates of the type commonly used in lithographic printing.
As may be appreciated from the above, the limitations of the prior art are as follows:
1. The apparatus of the referenced patent and patent application provide for direct transfer, without offset transfer capability. PA0 2. The intermediate member is required to have an electrically conductive working surface. PA0 3. A correct reading color proof or print cannot be produced selectively by direct transfer or offset transfer using the same apparatus without employing a transparency of the opposite sense, i.e. by using a negative transparency instead of a positive transparency. PA0 4. A single apparatus cannot selectively produce a correct reading color proof by direct transfer and/or offset transfer on the same kind of substrate as the printing substrate or stock.
The herein invention resolves the problems of the prior art as recited herein.